Code division multiplexing in spread spectrum communication systems is well known. In a spread spectrum communication system, a spreading code sequence is used to disperse the energy content of an original input data signal in the frequency spectrum. In order to transmit each data bit in a spread spectrum communication system, each individual data bit is first multiplied by the spreading code sequence in an exclusive OR gate. In a typical system, for a data bit of zero, the spreading code sequence itself is transmitted; while for a data bit of one, the inverse of the spreading code sequence is transmitted.
At the receiver, the spreading code sequence is detected by correlating the received spread spectrum signal with knowledge of the original spreading code sequence which was used at the transmitter. One method and apparatus for encoding transmitted data and correlating received data in a spread spectrum system can be found in a co-pending patent application entitled "DIFFERENTIAL CORRELATOR FOR SPREAD SPECTRUM COMMUNICATION SYSTEM", serial number 271,614, filed Sept. 15, 1988 and assigned to the same assignee as the assignee of the present invention.
The object in any multiplex communication system (such as a frequency division multiplex system or a time division multiplex system, as well as a code division multiplex system), is to provide for independent communication paths between individual devices without interference to neighboring devices, i.e., to establish distinct channels of communication without mutual interference.
Code division multiplex, or CDM, is a method for establishing such different distinct channels in a spread spectrum communication system. In a CDM communication system, different spreading code sequences are used to distinguish the different data communication channels. Each spread spectrum receiver correlates the received signal with a particular spreading code sequence, thereby establishing independent channels of communication, one channel for each distinct spread spectrum spreading code sequence used.
The individual bits of the final spread spectrum signal, consisting of the spreading code sequence combined with the message data, are called chips. As used herein, the term chip may also apply to the individual bits of the spreading code sequence particularly during the process of combining the spreading code sequence with the message data at the transmitter encoder, or in the process of separating the message data from the spreading code sequence at the receiver correlator.